The present invention relates to a web unwinder and more particularly to an improved web unwinder in which a position detecting means for measuring the diameter of a core does not interfere with the web roll assembly.
A web unwinder is used for feeding a web such as photographic film, paper, cloth, or the like drawn out from a web roll to a working line. In the web unwinder, a web roll assembly comprising a core and a web roll around the core is rotatably mounted. While the web is continuously drawn out, the amount of the web in the web roll is measured. When the amount of the web decreases to a predetermined first limit value, the web feeding speed is caused to decrease. Then, the feeding of the web is caused to stop when the amount of the remaining web decreases to a predetermined second limit value. After the stoppage of the web feeding, the web roll assembly is replaced with a new web roll assembly wound with a full web.
The amount of the remaining web in the web roll can be calculated from the winding thickness of the web roll. For this calculation, the diameters D and d of the web roll and the core, respectively, are measured. If the diameters d of all of the core were exactly the same, only the diameters D of the web roll would be measured. Actually, there is a variation in the values of the core diameter d even though the cores are of the same type. Therefore, the diameter d of each core must be precisely measured, especially when the web is very thin.
As is shown in FIG. 2, a web roll assembly 2 of the prior art comprises a core 4 and web roll 3 wound with a web 5 around the core 4. The measurement of the core diameter d is performed by using an optical position detector 6 facing the surface of the core 4. The measurement of the web roll diameter D is also performed by using another optical position detector (not shown in FIG. 2) facing one of the ends of the web roll 3. The optional position detector 6 comprises a light source such as a laser diode, and a line sensor 9. A light beam emitted from the light source is projected onto a surface of the core 4 through a lens 8a. Part of the light reflected from the surface of the core 4 is received by a line sensor 9 through a lens 8b. According to the position wherein the incident light is received by the line sensor 9, the line sensor 9 generates a position signal corresponding to the radius of the core 4. Based on the position signal, the core diameter d can be calculated as follows: previously a diameter d.sub.1 of a reference core is measured, for example, with a caliper. Next, the reference core and another core having an unknown diameter are measured with the optical position detector 6. When the position signal of the reference core corresponds to the radius r.sub.1 and the position signal of the core corresponds to the radius r.sub.2, the unknown diameter d.sub.2 of the core is given by EQU d.sub.2 =d.sub.1 +2(r.sub.1 -r.sub.2)
The above measurement is described in detail in U.S. Pat. No. 4,657,198.
In a measurement with such an optical position detector 6, the detector 6 is disposed as close as possible to the core 4. The reason for this is that it is desirable that the intensity of the light projected onto the surface of the web be greater, and the intensity of the incident light falling on the line sensor 9 is also greater. Furthermore, it is also a requirement that the optical axis 7a of the light beam be preferably perpendicular to the web surface as shown in FIG. 3. Accordingly the optical position detector 6 is required not to interfere with the web roll 3 even when the web roll has a large width of web 5 and a larger diameter of web roll.
In view of these circumstances, there has been proposed a core diameter measuring device which is disclosed in Japanese Utility Model Application No. 62-33969. The core diameter measuring device has a position detector which is slanted to the vertical line of the surface of the core 4 so as to be away from the side of the web roll 3. In this core diameter measuring device, there is still an unsolved problem in that the position detector interferes with the web roll which may have an undesirably great width of web. In order to solve this problem, another arrangement of the position detector 6 can be considered. That is, the position detector is mounted for movement with a moving member movable according to the width of the web. This method, however, leads to a larger and more complicated web unwinder. If the web roll assembly is set inaccurately, the position detector would interfere with the web roll and a position to be measured on the core would undesirably change.